Septal defects generally refer to a perforation or hole passing through a septum. A septum is a thin wall of muscle separating two cavities. Atrial septal defects ("ASD") are a common congenital cardiac abnormality. A large atrial septal defect can lead to enlargement of the right atrium and right ventricle. Also, a large atrial septal defect may require closure to prevent these complications. To close an ASD, open heart surgery has been used for decades. In such an operation, the patient's chest must be opened and heart temporarily bypassed. Then, the surgeon sutures the defect shut or if the defect is too large, a patch of biocompatible material is sewn in place to close the aperture.
In order to avoid the trauma and complications caused by open heart surgery, transcatheter techniques to close septal defects have been attempted. These techniques deliver an occlusion device through a catheter to the septal defect. The device is placed into the defect and permanently deployed.
A number of such transcatheter devices have been described. King et al. proposed such a device in U.S. Pat. No. 3,874,388. The King apparatus comprises a pair of complicated umbrellas. Each umbrella frame is made of stainless steel with each frame having six ribs to maintain each umbrella in an open position. Also the King apparatus has barbs at the ends of each rib to anchor the apparatus to the surrounding tissue. While the King apparatus marked an improvement over open-heart surgery, the umbrellas are difficult to unfold after passage through a catheter. Moreover, repositioning of the apparatus is almost impossible as the barbs prevent movement once they are anchored to the tissue. Finally, due to the large size of the device and its delivery catheter (23 French), the device is not appropriate for use with small children.
Another device, the Rashkind occluder provides a single umbrella closure apparatus with a delivery system which permitted use in small children. Rashkind's apparatus is a single disk umbrella-type closure device with barbed hooks similar to those of King. Rashkind deploys the single umbrella within the left atrium and then positions it against the left side of the atrial septum. As with King, the barbed hooks prevent disengagement of the device upon deployment. This results in poor centering capabilities and no re-positioning or removal.
Lock, et al., developed a "modified" double-umbrella occlusion apparatus. Lock's apparatus resembles a clamshell which is well known in the art. The arms of Lock's "clamshell" device are hinged to allow them to fold back against themselves. Also Lock's device is more compact than its predecessor's as it allows delivery in children weighing as little as 8 kilograms.
Sideris proposed an occlusion apparatus that combined a single umbrella occluder with a separate anchoring device. This invention is shown in U.S. Pat. No. 4,917,089 . Sideris'apparatus utilized a series of ribs to support an umbrella. A string connects the arms of the umbrella to a rhomboidally shaped anchor. This anchor includes an internal wire skeleton and a central piece of rubber. The anchor is positioned opposite the umbrella with a length of string limiting the movement of the device.
Marks, in U.S. Pat. No. 5,108,420, discloses is an occlusion apparatus in which a wire frame is folded and delivered to the aperture via a catheter. Marks uses a shape-retaining alloy such as nitinol to form its wire frame. Upon delivery, the wire is allowed to unfold to yield two planar members. The wire frame may be covered with a membrane or the frame by itself may occlude the aperture. The Marks apparatus is made from a thermal formulation of shape retaining nitinol alloy. This thermal formulation nitinol is difficult to generate within the appropriate transition temperature range (TTR) for the human body. Formulation of this nitinol is difficult because the range between full deployment at body temperature and zero deployment at room temperature is only 17.degree. C. To prevent early deployment of the frame during delivery, Marks infuses the catheter delivery system with a cool, saline bath in an attempt to keep the wire at room temperature during transport to the aperture.
Besides the above-mentioned delivery complications encountered by Marks, the structural integrity of the frame is less than desirable. The frame has multiple arms attached to a central loop and extending radially from this loop. Due to this multiple wire arm configuration, the arms might easily break at their connection point as the wire arms bear the majority of the stresses placed on the apparatus. Also, due to this configuration, the Marks apparatus does not resist loading well.
Marks also teaches that a membrane should be attached to the wire or the wires should be embedded in this membrane. By having an attached or embedded membrane, the wire may not be used to re-position the occluding device once deployed. In other words, Marks device is cumbersome because the wire cannot be manipulated within the membrane. Furthermore, since Mark's wire is body temperature activated, once deployed, it cannot be re-folded and withdrawn if not positioned properly.
Das, in U.S. Pat. No. 5,334,217, proposed an occlusion apparatus having a pair of disks. Each disk is attached to a wire frame with each wire frame being formulated of a shape retaining alloy in a super-elastic formulation. Das forms each frame from a single strand of wire. The wire is twisted to form a plurality of legs with the ends of the wire connected to each other. The device is folded upon itself and forced into a catheter for delivery. Once the catheter has reached the aperture, the distal frame is pushed out of the catheter and expanded into the cavity distal to the aperture. Then, the other frame is allowed to expand in the cavity proximal to the aperture. Also, Das attaches a membrane to each portion of the frame so the wire frame may not be independently removed and the device cannot be readily collapsed back into the delivery catheter to be removed or repositioned. Thus, an occlusion device with repositioning and retrieval capabilities is desirable.